A tap changer capable of tap changing under load is described, for example, in DE 40 09 038 A1 and the corresponding U.S. Pat. No. 5,128,605. In such tap changers, a vacuum interrupter for each phase is opened before a mechanical switch is shifted to prevent significant arcing at the mechanical switch.
In such tap changers it is a common practice to monitor mechanical contacts and in the past this has often proved to be a problem since the mechanical contacts are bathed in oil. As a consequence, vacuum interrupters have been provided and it is of course desirable to monitor the operation of the vacuum interrupters since, should a vacuum interrupter not open in a timely fashion and thus relieve the mechanical switching elements for the particular tap from the current, a high-power electric arc can form which can result in a burning away of the mechanical contacts. Frequent switching under arcing conditions can result in major damage to the tap changer or can create critical operating conditions which must be avoided.
The monitoring system of DE 40 09 038 A1 and U.S. Pat. No. 5,128,605 is described for a tap changer of the reactor switching type. In this known monitoring system, in the current feed of each vacuum interrupter there is a respective current converter which supplies an output voltage to a transmitter diode (LED) which is associated with a light waveguide (light-transmitting cable or fiber) or optical fiber, which delivers that optical signal to a respective receiver. The current converter is thus capable of signalling whether or not the respective branch which is monitored has a current flow. The monitoring system is thus capable of determining whether, at a critical point in time determined by the actuation of the mechanical switch, the current flow has been interrupted. It is important, for example, to know the status of the vacuum interrupter and to be certain that the current flow has been interrupted before the mechanical switch begins to shift. If at this latter point in time a current is still present in the vacuum interrupter for the given phase, a defect can be signaled for that phase, indicating that the vacuum interrupter has not opened.
This monitoring system has been found to be effective and has been used for years and, has been available from Rheinhausen Manufacturing Inc. as its type “RMV” load-type tap changer. That system, however, has a drawback in that the status information must be supplied by light waveguides which extend out of the oil-filled interior of the tap changer. The waveguides can alter the dielectric breakdown voltage within the tap changer and also require special seals.